Regioselective 1,3-dipolar cycloadditions of (1Z)-1-(arylmethylidene)-5,5-dimethyl-3-oxopyrazolidin-1-ium-2-azomethine imines to acetylenic dipolarophiles

The 5,5-dimethylpyrazolidin-3-one (4), prepared from ethyl 3-methylbut-2-en oate (3) and hydrazine hydrate, was treated with various substituted benzal dehydes 5a-i to give the corresponding (1Z)-1-(arylmethylidene)-5.5-dimethy l-3-oxopyrazolidin-1-ium-2-ide azomethine imines 6a-i. The I,3-dipolar cycl oaddition reactions of azomethine imines 6a-h with dimethyl acetylenedicarb oxylate (=dimethyl but-2-ynedioate; 7) afforded the corresponding dimethyl pyrazolo[1,2-a]pyrazoledicarboxylates 8a-h, while by cycloaddition of 6 wit h methyl propiolate (= methyl prop-2-ynoate; 9), regioisomeric methyl pyraz olo[1,2-a]-pyrazolemonocarboxylates 10 and 11 were obtained. The regioselec tivity of cycloadditions of azomethine imines Ga-i with methyl propiolate ( 9) was influenced by the substituents on the aryl residue. Thus, azomethine imines 6a-e derived from benzaldehydes 5a-e with a single substituent or w ithout a substituent at the ortho-positions in the aryl residue, led to mix tures of regioisomers 10a-e and 11a-e. Azomethine imines 6f-i derived from 2,6-disubstituted benzaldehydes 5f-i gave single regioisomers 10f-i.